1. Field of the Invention
The present invention relates to an image formation apparatus such as a copier, printer, facsimile apparatus or combinations of these, and to process cartridges installed therein. In particular, the present invention relates to an image formation apparatus and process cartridge that uses a trickle development system that suitably discharges degraded carrier from within the developing unit to outside the developing unit, and a cleanerless system that cleans the untransferred toner on the image support member in the developing unit.
2. Description of the Background Art
In the past a variety of technologies were disclosed to make the imaging units in image formation apparatuses of electronic photographic systems more durable. For example, disclosed in Japanese Unexamined Patent Application Publication No. 2002-62724 is a technology to make the imaging unit more durable by using the trickle development system. In more detail, toner and a small quantity of carrier are refilled and excess carrier is discharged from the development unit of a two-component development system in which a magnetic carrier is made to spike up by magnetic force on a developer support member of a developing roller, etc., (suitably called a “magnetic brush” hereinafter) and is made to contact the image support member of a photo-sensitive body, etc., thereby causing toner particles within the magnetic brush to contact the image support member. The intention of this technology was to make the imaging unit more durable by reducing the degradation of image quality caused by degraded magnetic carrier. Specifically, when continuously agitating the magnetic carrier in the development unit of a two component development system, foreign matter such as the parent resin of the toner particles and external additives adhere to the surface of the carrier, and the frictional electrification capacity of the carrier decreases in relation to the toner. Further, mechanical impact causes the coated film of magnetic carriers having a coated film to peel off, and toner is then prone to adhere. Trickle development systems that suitably discharge degraded carrier within the development unit to outside the development unit in this way may be expected to have the effect of reducing deterioration of image quality over time.
Meanwhile, disclosed in Japanese Unexamined Patent Application Publication No. 2002-278256 is a technology to make the imaging unit durable using a cleanerless system. In further detail, the imaging unit of the cleanerless system is not provided with a cleaning unit to mechanically recover with a cleaning blade, etc. untransferred toner on the image support member, specifically, toner that is not transferred to the transfer receiving material in the transfer process and remains on the image support member. The intention of this technology was to make the imaging unit more durable by reducing the abrasion from the image support member caused by contact with the cleaning blade, etc. Concretely, instead of using a cleaning blade, etc. to recover the untransferred toner in the imaging unit of the cleanerless system, many mechanisms are employed which recover and directly reuse untransferred toner in the developing unit. If these kinds of mechanisms are used in an imaging unit of a two-component development system, the untransferred toner recovery characteristics are improved by increasing the relative motional velocities of both opposing parts of the developer support member and the image support member. Consequently, a counter-contact development system is used in which the direction of motion of the image support member is the opposite to the direction of motion of the developer support member (magnetic brush).
Using either of the conventional image formation apparatuses described above, the trickle development system or the cleanerless system, can achieve improved durability of the apparatus. Consequently, it may be expected that further improved durability of the apparatus could be gained by combining the trickle development system with the cleanerless system. However, a variety of problems arise when actually combining the trickle development system and the cleanerless system.
As the result of relentless research, the inventors of the present application discovered the following facts.
First, the case of installing a trickle development system in an image formation apparatus pre-equipped with a cleanerless system will be considered. In a trickle development system, carriers with differing degrees of degradation are essentially mixed together in the interior of the developing unit. Concretely, new carrier just supplied to the development unit together with toner has little foreign material adhering to the surface thereof, and the capacity to cause frictional electrification of the toner is high. In contrast, carrier that has been agitated a long time in the development unit has a large amount of adhering foreign material, and the capacity to cause frictional electrification of the toner is low. Consequently, the distribution of the amount of carrier charge in the magnetic brush is broadened. Moreover, carrier with a large amount of adhering foreign matter has high electrical resistance. Accordingly, fluctuations of the amount of toner particle charge in the magnetic brush are prone to occur. In addition, the development electric field that is formed between the tip of the magnetic brush and the image support member is also prone to become uneven. Further, when the amount of adhering foreign material differs, differences arise in the responsiveness to the magnetic field of the carriers and in the flow characteristics of the carriers. As a result, differences arise in the flexibility and strength as a magnetic brush.
Consequently, when installing a trickle development system in an image formation apparatus pre-equipped with a cleanerless system, fluctuations of the strength of the magnetic field in the tips of the magnetic brush cause irregularities to arise in the capacity to electrostatically draw untransferred toner to the development unit side. In addition, fluctuations of the flexibility and strength of the magnetic brush cause irregularities to arise in the capacity to physically scrape off untransferred toner. Setting the conditions for recovering the untransferred toner by the magnetic brush is delicate, and if irregularities of the recovery capacity of the magnetic brush arise longitudinally, the degree of margin for uniformly recovering untransferred toner across the longitudinal direction is lost.
Next, the case of installing a cleanerless system in an image formation apparatus pre-equipped with a trickle development system will be considered. As described above, when the relative motional velocities of the opposing parts of the image support member and the developer support member have been increased in order to improve the untransferred toner recovery characteristics, the velocity at which the carrier collides with the image support member is heightened. This strengthens the impact when the toner particles on the tip of the magnetic brush collide with the untransferred toner and carrier adhering on the image support member, increasing the adhesion of toner particle parent resin and external additives onto the carrier. Moreover, the impact at the time of collision is prone to cause the coated film of carriers having a coated film to peel off. If degradation of the carrier progresses in this way, differences in carrier characteristics within the magnetic brush will broaden and it will not be possible to achieve uniform development.
Consequently, when installing a cleanerless system in an image formation apparatus pre-equipped with a trickle development system, degradation of the carrier is promoted, and the desired effect cannot be obtained unless the cycle of supplying fresh carrier and discharging degraded carrier is expedited.